In vehicle suspension systems for large trucks and trailers it is common to mount a number of axles to the trailer in order to support the vehicle load. One commonly employed suspension system includes three axles which are supported by a combination of leaf springs and fixed beams. In the prior known three axle suspension, two axles are mounted to equalizing beams which are rockable about a trunnion tube. The trunnion tube is in turn secured to the vehicle by a mounting bracket which is welded or bolted to the underside of the trailer or vehicle. The third axle of the suspension is supported by a leaf spring assembly which is suspended between the inner ends of the two equalizing beams. Such a three axle suspension is manufactured by the Neway Division of Lear Siegler, Inc., the Assignee of this application, as their TR-8750 suspension.
In order to provide a suspension which is capable of carrying a greater vehicle load than a three axle suspension, it has been known to mount a separate axle to a trailer already provided with a three axle suspension. In such an arrangement, the fourth axle is not interconnected with any of the remaining axles and thus no interdependent distribution of the load between the three axles and the fourth takes place. If the axles are subjected to an uneven load such as occurs when the vehicle travels over uneven terrain, one axle will be more greatly stressed than the others. Further, the fourth axle must be carefully adjusted so that it carries a load equal to the other three axles on even terrain. Other combinations of conventional axle assemblies may be used to mount four or more axles to a vehicle. For example, two two-axle, tandem assemblies may be mounted to a trailer to provide the desired load capacity. These combinations also require careful adjustments to equalize the load on all axles.
It is also known to separately mount four axles to a vehicle by the use of four separate leaf spring assemblies. Each leaf spring assembly is secured to the vehicle and carries an axle connection for mounting an axle to the vehicle. Each leaf spring assembly separately responds to the vehicle terrain and there is no interconnection between the leaf spring assemblies which would distribute the load equally over the axles. The independent operation of each assembly detracts from ride stability and does not add to the roll stability of the vehicle about its axis in the direction of travel. Roll stability of leaf spring suspensions is less than that for trailing arm suspensions. Also, each spring assembly has a plurality of moving parts all of which create friction when the springs deflect so as to detract from ride stability.
Thus, it can be seen that the prior known suspension assemblies which include four or more axles do not provide for interconnection between each axle which allows for even distribution of a load. Further, such prior known suspensions include a multiplicity of parts and are often constructed by assembling separate suspension systems having one or two axles, requiring careful adjustments.